1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method used for a scanner, a digital copying machine, and the like that read image information by scanning an original document with a CCD line sensor.
2. Description of the Related Art
As typical examples of a CCD line sensor used in a reducing optical system, there are a line sensor comprising a I-line line sensor, and a sensor comprising a 3-line line sensor formed of three line sensors on the surfaces of which are respectively disposed color filters in red (hereinafter, occasionally abbreviated to R), green (hereinafter, occasionally abbreviated to G), and blue (hereinafter, occasionally abbreviated to B).
The 1-line CCD line sensor is basically used to read a monochrome original document. When a color original document is read using the 1-line CCD line sensor, an adopted method is to provide three light sources having spectral characteristics of R, G, and B, which are three primary colors of light, so as to read image information of a color original document by separating the image information into color information of R, G, and B by sequentially lighting ON these light sources. There is another method by which a light source having the spectral characteristic of white is used, and color filters in R, G, and B are disposed in the optical path between the light source and the line sensor, so that color information going incident on the line sensor is separated by switching the color filters.
Meanwhile, the 3-line CCD line sensor is basically used to read a color original document. In this case, a light source having oscillation wavelengths of 400 nm to 700 nm and thereby having spectral characteristics that sufficiently cover a visible light region is used, and separation of color information of R, G, and B is performed using the color filters disposed on the surfaces of the respective line sensors.
In addition, in a case where a monochrome original document is read using the 3-line CCD line sensor, there are a method by which one output from the 3-line CCD line sensor, that is, an output of the CCD line sensor for G in general, is used with the purpose of reading a seal in vermillion in a reliable manner, and a method by which black and white information is generated using all the outputs of the 3-line CCD line sensor.
However, the methods in the related art for reading a color original document in a monochrome state possibly have inconveniences as follows. For example, in a case where a color original document is read by a typical monochrome scanner using a line sensor on the light-receiving surface of which is disposed no color filter, reflected light from the original document goes incident on the line sensor, which makes it possible to read a variance in luminance but makes it impossible to read information about colors. Hence, in a case where an original document is made of red characters on the base of blue, although it is affected by the spectral characteristic of the light source, blue and red cannot be distinguished from each other when the reflectance is the same, and they are handled as the same signal. This gives rise missing of information when a color original document is read by the monochrome scanner. When a copying operation to make print on a sheet of paper is performed using such a signal, there is a problem that missing of characters, an image, or the like occurs.
In addition, in a case where a color original document is copied in monochrome using the 3-line CCD line sensor formed of three CCD line sensors on the surfaces of which are respectively disposed color filters in red, green and blue, there may be a case, for example, where the color of characters and the color of the base become the same depending on the colors of the color original document, which gives rise to missing of information on the original document. In the case of a scanner, because image information is read by forming the image of reflected light from the original document on each line sensor, color information is reproduced in red, blue, and green, which are the three primary colors of light.
Further, there is a method by which a pseudo achromatic color is generated by adding up the wavelength regions of red, blue, and green, which are the color filters on the line sensors. In this case, it is possible to calculate monochrome information=(red information+blue information+green information)/3.
However, when this processing is used, for example, in a case where information is made of red characters on the base in blue, given that the outputs of the respective line sensors when reading the blue information of the base are (red:blue:green)=(0:255:0), and the outputs of the respective line sensors when reading the red character information are (red:blue:green)=(255:0:0), then,
(0+255+0)/3=85 when the blue base information is converted to monochrome, and
(255+0+0)/3=85 when the red character information is converted to monochrome.
It is therefore understood that when such a color original document is copied in monochrome, the signals become the same color.
Likewise, because the information having the same addition result of red, blue, and green is deemed as the same information, even the information having different balances (chromaticity) among red, blue, and green is handled as a signal for a copy in monochrome. Hence, when a color original document is copied in monochrome, there is a problem that missing of characters or an image occurs.
Similar techniques are disclosed in Official Gazette (A) JP-A-2003-274115 and Official Gazette (B) JP-A-11-187266. Official Gazette (A) describes a reading device using a 4-line line sensor but is silent about a correction of a monochrome signal using a color signal. Official Gazette (B) discloses a base removing method. However, it is configured so as to perform a density correction for RGB signals, and is different from the technique of the invention.